(1) Field of the Invention
This invention relates to a preparation process of hexamethylene diisocyanate (hereinafter abbreviated as "HDI" for the sake of brevity), and more specifically to a process for preparing HDI in accordance with the hexamethylenediamine hydrochloride (hereinafter abbreviated as "HDA.HCl" for the sake of brevity) process, in which HDI is advantageously obtained on an industrial scale by preparing HDA.HCl of a relatively high concentration and then reacting it with phosgene. HDI is used as a modifier for various products including non-yellowing coating formulations and its demand is progressively increasing in recent years.
(2) Prior Art of the Invention
HDI has heretofore been prepared by a one-step reaction or cold two-step process, in which hexamethylenediamine (hereinafter abbreviated as "HDA" for the sake of brevity) and phosgene are reacted directly to each other, or by a hydrochloride process in which HDA is first formed into an acid addition salt such as HDA.HCl and then phosgenating the same. Although the direct one-step reaction features shorter reaction time as a merit thereof, it is accompanied by a drawback that byproduct tar and chlorohexyl isocyanate (hereinafter abbreviated as "Cl-HI" for the sake of brevity) occur in large amounts. On the other hand, the occurrence of byproducts is suppressed in the hydrochloride process. The hydrochloride process is however accompanied by such drawbacks that the phosgenation reaction takes longer time and when the concentration of the starting amine is high upon preparation of HDA.HCl, the viscosity of the reaction mixture increases significantly in the course of the salt-forming reaction and HDA tends to remain unreacted, and when the reaction mixture is phosgenated, more byproducts such as urea, Cl-HI and the like are formed. Accordingly, the preparation of the hydrochloride is generally effected to obtain HDA.HCl at a concentration of about 10% or so and the phosgenation is usually conducted on HDA.HCl of such a concentration.
U.S. Patent Specification No. 3,424,780 discloses that in a reaction between HDA.HCl obtained in a usual manner and phosgene, the occurrence of byproducts can be successfully suppressed by using a solvent in a large amount as much as 20-30 times relative to HDA.HCl. It is however economical to use such a solvent as little as possible in view of the size of a reactor and the recovery of the solvent. In addition, British Patent Specification No. 1,146,664 discloses to prepare HDA.HCl efficiently by means of a falling-film salt-forming apparatus and then to phosgenate the salt. This process is however accompanied by a shortcoming that a special apparatus is indispensable for the salt-forming step.
It has also been reported, for example, in U.S. Patent Specification No. 3,544,611 that the reaction velocity may be increased, the reactants may be used at higher concentrations and the yield of the intended product may be improved, all, by the pressure phosgenation process. However, these reported processes are all concerned with direct phosgenation reactions, in which free HDA is used. The phosgenation reaction of free HDA proceeds faster than the hydrochloride process and more byproducts are formed as the concentrations of the reactants increase. With a view toward suppressing the occurrence of such byproducts, the phosgenation is effected under pressure. It is also disclosed in these specifications that the unreacted phosgene can be recovered with good efficiency when the reaction is conducted under pressure.